Semiconductor Nanocrystal Quantum Dot Synthesis Approaches Towards Large-Scale Industrial Production for Energy Applications Michael Z

CORE Metadata, citation and similar papers at core.ac.uk

Provided by Springer - Publisher Connector

Hu and Zhu Nanoscale Research Letters (2015) 10:469 DOI 10.1186/s11671-015-1166-y

NANO REVIEW Open Access Semiconductor Nanocrystal Quantum Dot Synthesis Approaches Towards Large-Scale Industrial Production for Energy Applications Michael Z. Hu* and Ting Zhu

Abstract

This paper reviews the experimental synthesis and engineering developments that focused on various green approaches and large-scale process production routes for quantum dots. Fundamental process engineering principles were illustrated. In relation to the small-scale hot injection method, our discussions focus on the non-injection route that could be scaled up with engineering stir-tank reactors. In addition, applications that demand to utilize quantum dots as “commodity” chemicals are discussed, including solar cells and solid-state lightings. Keywords: Quantum dots, QDs, Scale up, Synthesis, Production

Review exceeding the Bohr radius are not within our discussion Semiconductor nanocrystal quantum dots have attracted since their application deviates from the quantum tun- more and more interests in solar cell, solid-state lighting, ability from the quantum confinement effect. Also, the and biological labeling fields due to the unique size- discussion of synthesis only restricts to those quantum tunable light absorption and emission properties. The dots which have identified their applications in energy large quantity demands of high-quality quantum dots saving and utilization fields; materials with special for advanced energy applications require an industrial morphology but without confirmed properties suitable applicable production method. However, the current for those applications are not within the scope of this re- quantum dot (QD) synthesis methods can only fulfill the view. Metal oxide semiconductor such as ZnO, due to requirements of small-scale Research and Development their distinct properties and applications and the large (R&D) and biological sampling/imaging. Novel ap- amount of literatures available, will not be discussed as proaches of QD synthesis suitable for scale-up produc- well (readers may refer to Ref. [1] for additional information are thus essential for the commercialization of tion). Due to the aim of practical industry application, optoelectronic devices in the near future. This review the authors also would like to restrict the discussion paper discusses various synthesis methods for semicon- within the material system with high quality suitable for ductor nanocrystal quantum dots and their potential for energy applications, namely monodisperse with stable sur- future industrial scale-up. To do this, an insight view of face protection, decent optical or optoelectronic properties. the available synthesis mechanisms is also presented to There are a number of literatures available for the help in identifying the controlling factor in scale-up. synthesis of semiconductor nanocrystals or large-scale Here, the quantum dots are defined as the semicon- synthesis of nanoparticles. For example, Ref. [1] has pro- ductor nanocrystals with the quantum confinement. vided a comprehensive introduction of nanoparticle pro- Thus, the semiconductor nanoparticles with dimensions duction in large volume covering elemental metals and metalloids (semiconductors), chalcogenide II–VI and IV–VI semiconductors, III–V semiconductors, and ox- * Correspondence: [email protected] ides. The review will try to include the most recent Oak Ridge National Laboratory, Oak Ridge TN37831-6181, USA

© 2015 Hu and Zhu. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 2 of 15

updates not included in those reviews and discuss the control. As suggested, one ideal solution is the selection most feasible approaches towards large-scale production of an appropriate chemical to initiate nucleation at a in a practical point of view. desired temperature. This concept was proved in CdS Since the large-scale synthesis is aimed towards system by Cao and his collaborators [2]. To do this, advanced energy application, the potential candidate two nucleation initiators were introduced in a CdS must feature or have the potential to fulfill the fol- synthesis system, tetraethylthiuram disulfides and 2,2′- lowing requirements: dithiobisbenzothiazole, namely I1 and I2. The precursors and solvents used are the same as those used in 1. Easy processing the injection approach, with cadmium myristate and 2. High reproducibility sulfur as the two precursors, with myristic acid and 3. Low cost octadecene (ODE) serving as the capping ligand and 4. Environmental friendly solvent, respectively. The system was preheated to 120 °C under vacuum to obtain a clear solution. After that, the The most widely used QD synthesis method for high- temperature was slowly increased to 240 °C for nuclei to quality QD production is the hot injection approach. start. The size distribution narrows down until 4 min of There are a few number of review articles in the litera- growth and can be maintained for growth time of longer ture for the discussion of the injection method [47]. This than 12 h without new nucleation detected. The absorp- approach features a fast injection of precursor into a hot tion and photoluminescence (PL) of the CdS nanocrystals solution containing another precursor and has been are comparable to the product obtained by injection successfully achieved in various systems. However, the approaches [19, 90, 91]. Although a noticeable surface- reaction requires an instant homogeneous reaction trap emission is accompanied in the PL, it can be removed which is hard to achieve in large volume reaction vessels. by gentle fluorescent illuminations. The crystal structure This also brings an inherent complication and difficulties is found to be zinc blende instead of wurtzite from injec- in reproduction. Thus, the injection approach is not tion approaches. suitable for scale-up and large quantity synthesis. The nucleation initiators are not available for all material systems. A more practical way is the selection Non-Injection Organic Synthesis of appropriate precursors with appropriate activities. By The most challenging part of QD synthesis is the way to carefully controlling the available precursor concentra- initiate reaction. A monodispersed quantum dot needs tion and activity in the solution, the monodispersed QD the formation of a uniform nanocrystal nucleus in a very can also be obtained by a non-injection method. Ideal short period of time. This can be achieved by fast injec- precursors should exhibit significant reactivity transition tion of one precursor into the solution to start the fast near the desired growth temperature, in other words, and homogeneous nucleus formation. This has by far almost no reactivity below the point and very high re- been proved as the most successful approach in various activity above the point. Some of the features helping QD families. But the homogeneous reaction initiated by the transition include the melting/decomposition point fast injection is difficult to achieve in large volume reac- and solubility change under different temperatures. A tion vessels. The special requirement of fast and homo- few appropriate precursors and solvents selected are the geneous reaction is not suitable for industrial large-scale following: cadmium myristate and selenium powder in chemical vessel. ODE for CdSe spherical nanocrystals, cadmium myris- Due to the inherent limitation of injection approach, tate and tributylphophine selenice (TBPSe) in ODE for non-injection nanocrystal synthesis method has been CdSe nonspherical nanocrystals, cadmium octadecylpho- developed by various groups. Contrary to the injection sphonate and TBPTe for CdTe quantum dots [3]. Both approach, two different precursors are present in the CdSe and CdTe nanocrystals synthesized by this system simultaneously before the reaction starts at a cer- approach show sizes with a standard deviation of less tain temperature. As indicated in the injection approach, than 5 % [3]. The typical CdSe nanocrystals synthesized clear separation between nucleation and growth is desired have a PL quantum yield of 30–40 % and show a zinc for the production of monodispersed QD. The colloidal blende structure [3]. nanocrystals usually grow at an elevated temperature which Similar to the non-injection approaches above, requires a heating process with a certain temperature Yu and her collaborators developed various families of growth rate. The heating process could initiate active II–VI binary and ternary magic-sized nanocrystals precursors to nucleate partially and result in a concurrent (MSNs) [4–10]. The magic-sized nanocrystals are nucleation and growth [19]. On the other hand, for the considered thermodynamically favorable due to their precursors with too low activities, very little amount of special space structure such as core-shell configuration. nuclei will form and the growth rate can be too fast to More importantly, magic-sized nanocrystals are single- Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 3 of 15

sized ensembles and featuring extremely narrow narrow emission FWHM of 100 nm. Different lead and absorption/emission width. These special optical prop- sulfur sources have been used including lead oxide, lead erties are very attractive for solid-state lighting and acetate, bis(trimethylsilyl)sulfide ((TMS)2S), thioacetamide telecommunication applications. (TAA), and elemental sulfur (S). ODE is used as a reaction The II–VI binary systems Yu has developed include medium, and the reaction temperature is 30–120 °C. CdSe [4, 5], CdS [6], and PbS [7] magic-sized or regular- Besides the binary system mentioned above, non- sized quantum dots (MSQDs). For CdSe MSQD, the cad- injection approaches have been proved successfully also in mium source is cadmium acetate dehydrate (Cd(OAc)2 · a number of ternary systems such as CdTeSe magic-sized 2H2O) and elemental Se as the selenium source com- nanocrystals and CdSeS regular nanocrystals. Both ternary pound. The reaction temperature ranges from 120 to systems use similar sources as the binary systems men- 240 °C with three families of MSQD produced [5]. De- tioned above, fatty acids as ligands and ODE as reaction pending on the wavelength of bandgap absorption, they medium. MBTS was added in CdSeS case for the purpose are termed as Family 395,463, and 513 nm, respectively. of activating sulfur. Solid-state 13C cross polarization/ The synthesized QDs exhibit bright photoluminescence magic angle spinning (CP/MAS) NMR spectra combined with a full width at half maximum (FWHM) of 8 to with X-ray diffraction (XRD) confirmed the ternary 10 nm. The Stokes shift is also smaller than the regular alloyed system with uniformly distributed composition QD, e.g., Family 463 has an emission peak at 465 nm throughout the whole nanocrystals. rending a Stokes shift of only 2 nm. This small Stokes Besides the methods starting from two precursors as shift might indicate a dominant band-edge emission mentioned above, another non-injection approach uses a with very little trap emission involved. 1-Octadecene is single-source precursor to start the direct synthesis of used as the reaction medium and fatty acids are used as quantum dots. The earlier single-molecule precursor routes the capping ligands. The success of the non-injection ap- involve a hot injection of the precursor into TOPO solution proach lies in the low activity of the cadmium precursor. at high temperature thus not favorable for large-scale syn- The precursor is in the form of Cd(OAc)x–(OOC–(CH2)n– thesis [11, 12]. A recent work used lower temperature for CH3)2 − x and releases cadmium slowly for the reaction. A mixing and higher temperature for reaction to avoid the low acid-to-Cd feed molar ratio is important to keep the hot injection step [13]. (Me4N)4[S4Cd10(SPh)16]isthe low cadmium precursor activity. For different kinds of fatty single-source precursor, and it was added into hexadecyla- acids, an optimum window of ligand length is also present mine (HDA) at 80 °C under argon before temperature in- (C12–C18). A longer ligand is a better soluble in ODE and creased to 230 °C for reaction. Large quantities (>10 g/L) of weaker in binding with cadmium. Correspondingly, longer CdS QD could be produced by the method. A different ligands will increase the activity of Cd precursor and approach using inorganic clusters as single-source precur- monomer, thus leading to large MSQDs. Besides the sor allows the nucleation starting at relatively low nature of acids and acid-to-Cd ratio, the reaction temperature. The technique is based on the introduction of temperature and Cd-to-Se feed molar ratio are also inves- an inorganic metal-chalcogenide cluster into an alkylamine tigated as affecting factors. High Cd-to-Se ratio is found to solvent. In a typical reaction preparing CdSe, the precursor prevent the dissociation of the formed MSQDs. In one (Li)4[Cd10Se4(SPh)16] was added into HDA at 120 °C under word, a reaction temperature of 200–240 °C, a low acid- argon before the whole system was raised to 220–240 °C to-Cd ratio, a Cd-to-Se ratio, and a ligand length of C12 for the reaction [14]. The single-source precursor is stable to C18 are considered optimum for producing CdSe under ambient conditions, and the reactions can be readily MSQD with high nanocrystal concentration and PL QY. scaled to large quantities (1–50 g/L) without substantial ad- A similar reaction can be carried out in CdS system justment to the growth methodology. Via this approach, with elemental S as the sulfur source [6]. Myristic acid high-quality monodispersed CdSe, ZnSe, and CdSe/ZnS (MA) is used as a capping ligand, and 2,2′-dithiobisben- quantum dots have been prepared. One of the concerns for zothiazole (MBTS) is added to promote sulfur reactivity. this method is the design and the synthesis of the single- A series of experiment conditions are also investigated source precursor. The additional requirement of preparing to find out the optimized synthesis condition. With the a single-source precursor and the limited availability of a acid-to-Cd ratio fixed (2:1), a ratio of (1–2)Cd/1S and suitable precursor for different material systems might (8–32)S/1MBTS, a growth temperature of 220–350 °C is restrict its further development. determined to be ideal for QD growth. The obtained CdS QD has a FWHM of 17–22 nm and a quantum Synthesis Mechanisms yield up to 30 %. To achieve the scale-up synthesis of nanocrystals A similar non-injection and low temperature approach from current bench-based synthesis, it is important to was also used for PbS QD synthesis [7]. The obtained PbS understand the nanocrystal formation mechanism and QD has a bandgap emission from 600 to 900 nm with a the controlling factors. This could effectively form the Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 4 of 15

guideline for the large-scale engineering production of ensemble in pure form Family 463 (named by its bandgap nanocrystals. absorption peak). For the regular QD synthesis, detailed mechanism There are still arguments about the intrinsic properties investigations have been carried out [15, 19]. Usually, of magic-sized quantum dots. Different morphologies have the QD formation contains the nucleation and growth been observed under high-resolution TEM including dis- process. The burst of nucleation is followed by a persed particles and sheet structures. Magic-sized “cluster,” supersaturation of monomers. The growth of nuclei is “platelets” [17] or “nanosheet” [18], and “quantum dot” then followed by consuming the additional monomers have been used by different people, and the discrepancy present in the system. Depending on the different needs to be justified by further experiment. monomer concentration, QD with different shapes can Some theoretical calculations have been carried out to be grown with different size distributions. Both ther- prove that nanoclusters with magic numbers do exist modynamics and kinetics contribute to the nanocrystal with lower potential thus is thermodynamically more growth. For example, nanocrystals with low aspect ra- stable. So, it is plausible to assume that different magic- tios are obtained in the slow growth limit under sized clusters have local minima in chemical potentials. thermodynamic control while the nanocrystals with Previous investigations have noticed that magic-sized highly anisotropic shapes require a kinetic growth re- nanoclusters are frequently observed in the nucleation gime.AsAlivisatosandhiscolleaguesproposed,ata stage during the synthesis of elongated nanocrystals. The relatively low monomer concentration, nearly round formation of magic-sized nanoclusters originates from nanocrystals are formed with a relatively large size dis- their local minimum chemical potential because of the tribution. Under that regime, due to the depletion of closed-shell configurations. As Peng and his collabora- smaller particles and the continuous growth of larger tors suggested, magic-sized nanoclusters have local mini- particles, a size “defocusing” or “Ostwald ripening” is ob- mum chemical potential and form a local energy “well” served. When the monomer concentration increases, in the figure of chemical potential vs the size [19]. The nanocrystals with low aspect ratio are still observed with a formation of magic-sized nanoclusters happens under a narrower size distribution. This size “focusing” process relatively high chemical potential and they are only is due to the faster growth rate of smaller particles than stable at relatively high monomer concentrations due to larger particles which finally results in a monodispersed their extremely small sizes. It is suggested that the nanocrystal ensemble. In the two regimes mentioned, magic-sized nanoclusters can undergo two pathways theequilibriumshapeofnanocrystalshasalowaspect after the formation. With relatively high monomer con- ratio since this minimized the surface area as well as centration present in the reaction solution, it can grow the small energy difference between different facets. into regular nanocrystals with larger size by “tunneling” Since the growth rate of a facet depends exponentially through the lower thermodynamic barrier. On the other on the surface energy, the high-energy facets will grow hand, it may decompose to monomers by “tunneling” faster than low-energy facets. So when the monomer thorough the high barrier on the reverse direction. This concentration continues to increase, kinetics will begin process is highly favored with a lower monomer concen- to control nanocrystal growth resulting nanocrystals tration present in the solution. with highly anisotropic shapes. It has also been observed that magic-sized clusters On the contrary, only nucleation seems to be involved are formed only at the high monomer chemical po- in magic-sized nanocrystal formation. No growth in size tentials needed to form the rod-shaped nanocrystals, can be observed after homogeneous nucleation of thus their chemical potential corresponds to local nanocrystal since the absorption peak position is fixed minima in the progression from precursors to final ever since it appears. We believe that the magic-sized nanorods. QD formation is thermodynamically driven rather than Jiang and Kelly tried to propose a mechanism to kinetically controlled. Depending on the available ex- explain the formation of magic-sized clusters (MSCs) perimental data, Yu and Hu have proposed that the and to fit the current available experimental data [20]. formation of magic-sized QD is mainly determined by A number of species are discussed in the system includ- the thermodynamic equilibrium with little effect of ing the nuclei, the magic-sized clusters, nanorods (repre- chemical reaction kinetics involved [16]. By carefully senting nanocrystals with highly anisotropic shapes), and controlling the equilibrium between precursors and nanospheres (nanocrystals with low aspect ratio). Instead nanocrystal products, different families of magic-sized of considering magic-sized cluster as a candidate of QD and regular QD can be obtained. For example, a “critical size nuclei,” they differentiate the magic-sized reaction window of 2.7MA-4Cd-1Se molar ratio, a Se clusters from the regular nuclei and proposed a chem- concentration of 10 mmol/kg, and a reaction temperature ical potential relation as μMSC (magic-sized cluster) > of 200–240 °C could produce a magic-sized nanocrystal μN (nuclei) > μNR (nanorod) > μNS (nanosphere). Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 5 of 15

The central feature of the mechanism is the fast equi- inside with connections to the cage. Thus, a selection of librium between monomers and the magic-sized clusters. a highly symmetric cage with the right size of core will The magic-sized clusters can only form from monomers impose stringent restrictions for forming a stable nano- and solve back without the third possibility to form structure. By calculation, this novel 3D core-cage struc- regular nuclei/particles. The monomer concentration ture favors to take specific atom numbers to maximize has a saturation value of M0. The magic-sized clusters the binding energy. For example, the smallest polar can only form when the monomer concentration ex- cages with the highest possible symmetry (octahedral) ceeds M0 and will dissolve back when it is lower than have the number of n = 12, 16, and 28, in which (CdSe)5 the saturation value. The condensation and dissolution and (CdSe)6 core fits well into the (CdSe)28 cage to processes happen at a very fast rate so that magic-sized form an extremely stable network. The theoretical pre- clusters serve as a reservoir for monomers. In this pro- diction fits well with the experiment results including posed mechanism, the possibility of magic-sized clusters time-of-flight mass spectra, extended X-ray absorption serving as intermediate between monomer and regular fine structure (EXAFS), analysis and AFM estimate. This nanorods is excluded. calculation also predicts the similar “magic number” Based on the chemical potential relation indicated behavior in other II–VI compounds while is less stable above, several monomer concentration regimes can be for III–V nanoparticles. (One interesting phenomenon identified. When the monomer concentration exceeds they observed is also the appearance of regular QD by the saturation value M0, the additional monomer will simply heating MSQD to a higher temperature, the ab- form magic-sized clusters to keep monomer concen- sorption peak of MSQD remains without shift although tration within the saturation value M0.Inthisregime, the intensity decreases. This indicates the regular QD μM (monomer chemical potential) = μMSC > μN,only and MSQD are thermodynamically convertible without magic-sized clusters can be formed and they are in the aid of other chemicals. The presence of an inter- equilibrium with monomers. When the monomer concen- mediate state is still unknown.) tration decreases below M0, no magic-sized clusters can be formed and the existing clusters will dissolve back to Solvothermal Synthesis (organic medium) of Semiconductor form monomers. At the same time, regular nanocrystal Nanocrystal Quantum Dots nuclei begin to form (μMSC > μM > μN). When the Solvothermal method (in aqueous medium called monomer concentration continues to decrease (μN > hydrothermal) has been extensively utilized for nano- μM), no nucleation can occur and the additional particle synthesis. However, the earlier efforts to monomer will attach to the available nuclei to continue synthesize semiconductor quantum dots are largely re- the particle growth. If the monomer concentration stricted by the inferior product properties. Also, large continues to decrease, the reaction mechanism is iden- amount of works generated nanoparticles without tical to regular nanocrystal formation as explained quantum confinement, without tight control of particle above, with rod to sphere and Ostwald ripening (size size and morphology. Ref. [1] has provided an intro- “defocusing”)expected. duction until the year of 2004. The above mechanism for magic-sized nanocluster Recently, there are a number of efforts reporting the formation is identical when the monomer concentra- solvothermal synthesis of quantum dots in organic sol- tion falls into the regular nanocrystal nucleation/for- vents. CdS [22], CdSe and CdSe/CdS core/shell [23], mation regime. The formation of magic-sized clusters PbSe [24], InP [25], and CuInS2 [26, 27] quantum dots is due to the fairly high monomer concentration ex- have been reported via the approach. Solvothermal could ceeding the “kinetic control” regime. The biggest dif- provide evaluated temperature and pressure thus gener- ference between the two proposed mechanisms lies in ates unique synthesis condition for nanocrystal growth. the possibilities of magic-sized nanoclusters directly The solvothermal method employs similar synthesis towards regular nanocrystals with larger size. Further method as the normal batch synthesis, usually starting well-designed experiments are expected to help eluci- from the mixing of two individual precursors at lower dating the puzzle. temperature, then increases to the desired growth Theoretical calculations have been carried out to help temperature in a sealed autoclave for the nanocrystal understanding the MSQD forming mechanism. First- production. In a typical solvothermal growth of PbSe principle calculations using ultrasoft pseudopotentials quantum dots [24], Pb precursor was prepared by dis- and the generalized gradient approximation show that solving Pb(CH3COO)2 ·3H2O in octadecylamine to form (CdSe)n to be cage-like polyhedral [21]. The Cd and Se a clear solution at 80 °C. Se powder was rapidly added ions will connect alternatively to form zigzag networks into Pb precursor for a rigorous stir for ~10 min. The composed of four- and six-membered rings. Further- mixture was then sealed in a Teflon-lined stainless steel more, the cage can be stabilized by filling with a core autoclave and maintained at 200 °C for 1.5 h. Different Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 6 of 15

sizes of PbSe nanocrystals could be obtained by varying alloyed [45] systems. The hydrothermal synthesis also the initial Pb-to-Se ratio. starts from two precursors containing the cation and Although the synthesized quantum dots show narrow anion species, with mixing and heating towards the size distribution and quantum confined absorption growth temperature for quantum dot production. In a spectra, the reported photoluminescence efficiency is typical hydrothermal synthesis of CdTe quantum dot, still unknown or lower than those reported from or- NaBH4 was used to react with tellurium in DI water to ganometallic or its modified method. A PL quantum form the NaHTe as Te precursor. CdCl2 and thiol lig- yield (QY) of solvothermal synthesized CdSe is around and N-acetyl-L-cysteine (NAC; as the surfactant) were 5–10 % and could reach 18–40 % after a CdS shell was dissolved in DI water to form Cd precursor. The pH added [23]. This indicates a lower quality of quantum value of Cd precursor needs to be adjusted to 9.5 by dots possibly originated from higher defects level. stepwise addition of NaOH at 4 °C. The NaHTe solution was then added into the above Cd precursor at Aqueous and Hydrothermal Synthesis of Quantum Dots 0 °C with rigorous stirring, and the mixture was sub- Quantum dots made from aqueous synthesis is espe- sequently loaded into a Teflon-lined stainless steel cially attractive for biological application due to their autoclave. CdTe quantum dots will form by heating compatibility with water. Also compared to the the autoclave at 200 °C for ~1 h. Unlike the reaction organic-based synthesis, aqueous synthesis is cheaper, in organic medium, pH value is essential for aqueous- less toxic and more environmental friendly. Unfortu- based hydrothermal reaction and needs tight control to nately, the quality of the as-prepared quantum dots achieve desirable results. Also, the reaction temperature has relatively low quantum yield and large size distri- and precursor ratios are considered important parameters bution compared to organic approach samples. The to affect quantum dot formation as well. additional post-synthesis treatments such as size- Compared to normal aqueous synthesis, hydrothermal selective precipitation, selective photochemical etch- method could provide a higher reaction temperature ing, and surface modifications could further improve exceeding the boiling point of water and a higher pres- the quantum dot quality. However, those treatments sure which could favor the formation of nanocrystal. add additional complexity and cost thus are not desir- The quality of the synthesized quantum dots has im- able for large-scale production. proved significantly in recent years. The PL quantum Representative publications about aqueous synthesis of yield, as an important parameter to evaluate quantum QD include the material systems of CdSe [28], CdTe dot quality, is reported to be 68 % for CdTeS alloyed [29, 30], CdTe/ZnTe [31], CdTe/CdS [32], CdTe/CdSe QD [45], 45–64 % for CdTe/CdS, 27.4 % for CdTe [33], ZnS [34], ZnSe and Zn1 − xCdxSe alloyed [35], [41], and 44.2 % for CdTe/CdSe QD [44] for hydro- and ZnSe(S) alloyed [36]. Thiols and thioalkyl acids in- thermal synthesis approach. The FWHM, indicating cluding thioglycolic acid (TGA) and 3-mercaptopropionic the size distribution of the QD, is reported to be acid (MPA) are popular stabilizers used for aqueous syn- ~50 nm for CdTeS alloyed QD [45], ~40–50 nm for thesized quantum dots. Most of the aqueous synthesis CdTe/CdS [43], 40–80 nm for CdTe [41], and ~80 nm utilized the mixing of two precursors containing the re- for CdTe/CdSe [44]. Although the best value of the spective anions and cations to react at high temperature. QD yield is comparable to the parameters via organic The reaction mechanism follows the similar discussion as approach, the size distribution is still wider with fur- the organometallic approach featuring the nucleation and ther optimization needed. growth stages [37]. A different strategy developed by Li Hydrothermal and aqueous synthesis of nanocrystal utilized a water–oil heterogeneous system to synthesize quantum dots is relatively new compared to the well- nanocrystal and quantum dots [38]. The reaction is based developed organometallic synthesis and its derived on a general phase transfer and separation mechanism method. They have the intrinsic advantages in terms of [39]. Synthesis of CdSe [38, 40], ZnSe, ZnxCd1 − xSe [38], lower cost, lower toxicity, and “greener” processing. Al- CdS, PbS, and ZnS [39] quantum dots has been reported though attractive material properties such as narrow size from the methodology. distribution and high photoluminescence comparable to Hydrothermal synthesis uses water instead of organic organic synthesized quantum dots have been reported, solvents as the reaction medium. Although numerous relatively rare device applications have been explored. papers report the hydrothermal synthesis of nanoparti- The real application in energy field requires a more cles, few of them are within the quantum dot category complementary investigation with close interaction with with desired properties needed for applications. The device scientists needed. The formation of functional high-quality quantum dots synthesized by hydrothermal thin film and the corresponding device performance method can be found in CdTe [37, 41, 42], CdTe/CdS need further evaluations before concluding them as feas- core/shell [43], CdTe/CdSe core/shell [44], and CdTeS ible approaches for energy applications. Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 7 of 15

Possible Approaches Towards Large-Scale Synthesis of was used to maintain the inside atmosphere. To obtain a Semiconductor Nanocrystal Quantum Dots (microwave high-quality CdSe nanocrystal, the authors have investi- and flow-through) gated a number of variables. It is determined to be cru- Due to the increased demand of quantum dots for vari- cial to control a desired reaction temperature at the ous applications, scientists have begun to explore the mixing point of the starting liquids in the reactor. A large-scale synthesis of nanocrystal quantum dots by relatively high reaction temperature of 350 °C is neces- various means. Although there are a number of compan- sary for obtaining high-quality nanocrystal with high ies capable of generating a large amount of quantum photoluminescence efficiency. Also, the quality of com- dots, they are not discussed here due to the limit of their mercial TOPO affects the product and adding a small experiment details. All the discussions below are based ratio of TDPA helps to control the nanocrystal forma- on published journal papers and patents available. tion. After the optimization, up to 13 g/h production Compared to general chemistry, chemical engineer- rate was observed to last for at least 1 h. The bandwidth ing has accumulated extensive theoretical and experi- and the luminescence intensity of the nanocrystal were ment experience in large-scale synthesis. It is wise to comparable to batch reaction using the similar approach apply chemical engineering guideline to guide our be- [47, 48]. There are also a number of works reporting the havior in large-scale synthesis. Most of the syntheses synthesis of quantum dots via the microfluid reactor mentioned above are categorized as heterogeneous re- [51–53]. Although much smaller in size, the “flow- actions. In these kinds of reactions, the temperature, through” concept could still serve as a useful hint for pressure, and compositions are obviously important the future design of flow reactors. variables. Besides that, the momentum transport, en- Besides the continuous flow reactor, microwave syn- ergy (heat) transport, and mass transport are also key thesis is also a plausible approach towards large-scale variables for large-scale industrial synthesis. The un- synthesis. Microwave synthesis features simple operation derstanding and control of the above parameters are and potential of large-scale synthesis [54]. The micro- essential to obtain high-quality nanocrystal quantum wave synthesis has enabled production of high-quality dots as well as achieving higher yield. colloidal semiconductor nanocrystal both in organic and Selection of reactors is important to control the qual- aqueous media. ity and quantity of final products. At initial research For an aqueous medium reaction, a number of mono- stage, batch reaction is obviously the best choice in dispersed nanocrystals including CdTe [55], CdTe/CdS/ terms of easy processing and easy control. After the full ZnS [56], CdSe [57], ZnSe(S) alloyed [58], and CdSe(S) understanding of reaction mechanism and proof of high- alloyed [59] have been successfully synthesized by micro- quality nanocrystal production ability, a continuous wave or microwave-assisted synthesis. In a typical reaction process will be more preferable for higher production of preparing CdTe nanocrystal [56], the CdTe precursor rate and real industrial production. Large-scale produc- solution was prepared by adding freshly prepared NaHTe tion of semiconductor nanocrystal by a continuous flow solution to a N2-saturated CdCl2 solution (pH = 8.4) in reactor has been demonstrated by scientists from Japan the presence of the stabilizer 3-mercaptopropionic (MPA). [46]. The reaction recipe followed the previous batch The precursor solution was loaded into a vitreous ves- synthesis using trioctylphosphine oxide (TOPO) as the sel and heated at 100 °C for 1 min under microwave ir- capping organic ligand and the high temperature reac- radiation before a high-quality nanocrystal was obtained. tion solvent [47–50]. The continuous flow reactor is Some of the best quantum dots synthesized by this ap- composed of tanks of two starting liquids (ligand TOPO proach possess a high photoluminescence quantum yield and Cd/Se stock solution, respectively), two feeding as well as excellent photostability and favorable biocom- pumps, two feeding pipes, one reactor, one heating zone, patibility [56]. one condenser, and a collecting glass bottle. The reactor Other efforts have been put on colloidal synthesis of is made of stainless steel and is equipped with a static nanocrystal in organic medium. The synthesis method mixer (screw) for enhancing the mixing of the starting and recipe are similar to the previous reports with the liquids. Starting from the feed pump, the starting liquids microwave heating replacing the conventional heating. (TOPO and the Cd/Se stock solution) went through the High-quality monodispersed InGaP, InP and CdSe [60], feed pipes for preheating before they were continuously CdTe [61], CdS [62], and CdSe/ZnS [63] nanocrystals fed into the reactor from the feed pumps and pipes. The have been reported by this method. Microwave dielectric heating zone is equipped with stainless steel pipes for heating enhances not only the rate of formation but also desired reaction time and temperature. The product was the material quality and the size distribution. The final cooled to 70–80 °C (above the melting point of TOPO) quality of the microwave-generated materials depends by the condenser and finally collected in the collecting on the reactant choice, the applied power, the reaction glass bottle. In the whole reaction process, dry argon time, and the temperature. Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 8 of 15

In this approach, Strouse and his collaborators empha- based solid-state lighting, Ref. [69] provides a detailed sized the “specific microwave effect” which will select- introduction. As a result, this part is not intended to pro- ively heat certain compounds in the system due to the vide a detailed introduction of quantum dot-based opto- different polarization of different components. To bene- electronic devices. On the other hand, the authors try to fit from the effect, the reactive precursors have to be integrate the material property with the target application strong microwave absorbers and the solvent should have thus provides guideline for the optimization of quantum no or weak absorption to the microwave. In that case, dots and the future large-scale synthesis in industry. the polarizable reacting precursor will selectively absorb most of the microwave energy to overcome activation Solar Cells barriers and start the nucleation. Inspection of the cad- Solar cells are devices to convert sunlight to electricity mium source proved the trend: only the polarizable by photovoltaic effects. Multiple generations of solar starting precursors, cadmium stearate (CdSA), CdO, and cells have been developed since the 1940s. Solar cells CdNO3 produced nanocrystals while ionic CdCl2 had no made from crystalline silicon and GaAs show outstand- nanocrystal produced. ing performances but are limited by the high cost of ma- The use of microwave irradiation has several unique ad- terial and fabrication. Thin film solar cells made from vantages compared to convective heating. It can selectively CdTe and CIGS are promising but still suffer from cost heat the target precursor, has good reproducibility, and issues. Other interesting systems include dye-sensitized most importantly is compatible to non-injection approach and organic solar cells which benefit from their lower and could be coupled with a continuous process for large- fabrication cost and larger flexibility. scale production. In Strouse’s work, they have introduced The original driving force to use quantum dot comes a near continuous reaction called “stopped flow” approach from the expectation for quantum dot to generate mul- for nanocrystal synthesis [61]. In such kind of synthesis tiple excitons upon absorbing a single photon. This would for CdSe nanocrystal, the two precursors TOPSe and possibly boost the energy conversion efficiency beyond CdSA were delivered into the reaction chamber from the traditional Shockley and Queisser limit for silicon separate sources and heated to 190 °C for 4 h to complete solar cells [70]. Besides that, quantum dot combines the the reaction. After the reaction, the mixture-containing benefits of inorganic and organic materials. Compared to product is pumped out to a sealed vessel while the reac- conventional inorganic semiconductors, it has low cost, is tion can be continued by pumping new reaching precur- solution-based, has low temperature processing, is com- sors into the reaction vessel. This stop-flow design has patible to flexible and large area substrates, and has a demonstrated the yield of nanocrystal for ~650 mg/h. larger absorption cross section so that it needs much thin- ner layer to achieve complete absorption. Compared to Application of Semiconductor Nanocrystal in Energy organic semiconductors, it features enhanced charge sep- Utilization aration efficiency, balanced carrier transport properties, From the introduction of quantum dots [64], they are higher PL quantum efficiency (so that more excitons can closely related to semiconductor devices due to their be generated), and better chemical stability. Besides that, special quantum confinement properties. The current some quantum dot systems (e.g., PbSe) could cover infra- worldwide energy shortage requires renewable energies red spectrum which is barely utilized by the current avail- and energy-efficient techniques. Quantum dots are per- able solar cells. fect candidates for answering the challenges in terms of In the solar cell, the generation of electricity involves the their potential application in solar cell and solid-state processes of light absorption, exciton generation and diffu- lighting/display. Compared to the other applications of sion, charge separation, transportation, and collection. In quantum dots such as medical labeling, the energy-related most of the quantum dot-based solar cells, quantum dots applications require much more amount of quantum dots. not only serve as a light-harvesting material but also play Thus, the scale-up synthesis of quantum dots aims mainly multiple roles to help in charge separation and transporta- to the energy utilization, and the production has to be tion. Three types of solar cells have been reported in this shaped in order to fulfill the requirement for energy appli- category: quantum dot/metal Schottky junction solar cell, cations. The quantum dot application in energy field cur- polymer/quantum dot hybrid solar cell, and quantum dot/ rently falls in the two main categories: solar cell and LEDs. quantum dot D/A solar cell. There are plenty of review papers reviewing the area. For The simplest structure used for quantum dot-based example, Talapin and his collaborators recently provided a solar cell is the Schottky junction structure. The Schottky comprehensive review of colloidal nanocrystals for elec- barrier is made by contacting the quantum dot layer with tronic and optoelectronic applications [65] during this a metal electrode. The other side of the quantum dot layer manuscript preparation. For quantum dot-based solar cell, is contacted with ITO layer for the hole transport. Such readers may refer to Refs. [66–68]. For quantum dot- kind of device features easy structure and processing. Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 9 of 15

Several devices made from the structure have been re- device performance reported. The best material system ported based on PbS [71, 72], PbSe [73], and PbSxSe1 − x utilizes CdSe nanorod and P3HT and has achieved higher [74] nanocrystal systems. Due to the small bandgap of the power conversion efficiency. two nanocrystals, those devices are able to utilize infrared Due to stability concern of polymer/organic materials, part of the solar energy pretty effectively. The nanocrystal efforts have been put on developing pure inorganic thickness ranges from 65 nm to a few hundred nanome- quantum dot solar cell. One representative work has been ters. A ligand exchange process is reported to improve the done by Alivisatos and his colleagues. They use a CdSe/ nanocrystal conductivity and device performance. The CdTe nanorod system and a bilayer heterojunction struc- nanocrystal layer is made either by spin coating [71] or ture [76]. The CdSe and CdTe nanorods by their energy layer-by-layer dip coating [72, 73] techniques. The power alignment form a donor/acceptor pair. The films of CdTe conversion efficiency has been reported to be 2.1 % for and then CdSe were sequentially spin casted on ITO glass PbSe [73], 2 % for PbS [72], and 3.3 % for PbSxSe1 − x- coated with 2 Å alumina. CdTe and CdSe will serve to based devices [74]. More importantly, those devices transport holes and electrons, respectively. The charge can effectively collect infrared light with a monochro- separation happens not only at the donor/acceptor inter- matic power conversion efficiency (MPCE) of 4.2 % face but also along the length of the nanorod since little reported in the infrared [74]. Notice that the PbS quantum confinement is applied on the direction. Free quantum dot used in the Schottky junction structure charge carriers can be generated all along the thin film. is determined to be p-type owing to the high ratio of The power conversion efficiency is reported to be 2.9 %. hole-to-electron mobility [75]. Most recent develop- Without the organic materials involved, the device shows ment on infrared absorbing quantum dots and solar a better stability in the air. cells is updated in review articles [92, 93]. Quantum dots have also been widely used in solar Since semiconductor nanocrystals are good hole cells for sensitization. Semiconductors with visible light transport materials but not good at electron transport, absorption can serve as sensitizers to transfer electrons a quantum only layer will likely have an unbalanced to large bandgap semiconductors such as TiO2 and electron/hole transport and restrict the final device per- SnO2. People have built up such kind of solar cells simi- formance. To solve the issue, a quantum dot/conju- lar to dye-sensitized solar cells. A detailed review is gated polymer mixture layer is proposed to serve as the available by Kamat [77]. A power conversion efficiency active layer. P3HT and MEHPPV are the two most of 1 % has been reported for CdSe-sensitized TiO2 solar popular polymer materials to be selected due to their cell with a cobalt (II/III)-based redox system [78]. superior electron transport and light-harvesting cap- abilities. The blend facilitates the charge separation and More Discussion of Different QD Solar Cell Developments the generation of photocurrent. There are also other solar cell devices utilizing semicon- Two structures have been widely used in quantum ductor nanoparticles. They are not discussed here either dot/conjugated polymer blend solar cell: bilayer config- because the particle size exceeds the Bohr radius so no uration of n-type quantum dots and p-type conjugated quantum confinement is affected or because the nano- polymer and a so-called “bulk heterojunction” structure. particles are sintered and used as bulk materials. Since the generated excitons (upon light absorption) Theoretically, a suitable solar cell material system has have a diffusion length of ~10 nm, the bilayer configur- to satisfy the following requirements: a bandgap match- ation will have to limit the thickness of each active layer ing the solar spectrum, high electron and hole mobility, less than 20 nm for an effective charge carrier collection. long lifetime of charge carriers, good chemical stability, This, on the contrary, will restrict the light absorption. relative abundance of the composites, and relatively low A solution is to build an interconnected network of melting points. quantum dot and conjugated polymer called “bulk het- For the current applications, a few concerns need to erojunction” structure. In such kind of structure, donor be considered for quantum dots to be used in solar cells. and acceptor domains have the range close to the ex- First of all, the quantum dot should have strong absorp- citon diffusion distance so that most excitons can reach tion starting from infrared or visible. A quantum dot the donor and acceptor interface in a thicker device. system with too large bandgap will not be able to utilize Meanwhile, percolation pathways are required in both most parts of the solar spectrum while systems with too donor and acceptor domains for charge carriers to be small bandgap will have trouble with limited open- transported in each individual domain. An interconnected circuit voltage. Second, the quantum dot should have network meeting the two requirements could achieve bet- high electron mobility and long charge carrier lifetime ter light collection, charge separation, and transportation to ensure efficient charge transportation. Although the in a much thicker device than bilayer structure device. intrinsic property of the colloidal quantum dots deter- Devices with bulk heterojunction structures have better mines the transport property, surface ligands need to Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 10 of 15

have more concern in the practical experiment design. The most attracting optical property of quantum dot Surface ligands are required to protect quantum dots, in lighting is its narrow band emission. The FWHM of prevent agglomeration, and passivate surface from the the emission peak (also the first excitation peak of ab- solution to thin film form. In the device, appropriate li- sorption spectrum) is related to the size distribution of gands help to reduce the surface defects which will trap the obtained quantum dot. Regular quantum dots with the charge carriers and reduce the charge transportation appropriate size selection process could have a size dis- process. On the other hand, surface ligands are less con- tribution corresponding to a FWHM of ~20 nm while ductive or insulating materials which have negative effects the MSQD has a FWHM as narrow as 10 nm. on the device conductivity. To reduce that, appropriate Currently, quantum dot application in solid-state light- ligands need to be selected during synthesis. The best ing mainly lies in the phosphor usage in white LED appli- ligands should be able to be removed upon heating or UV cations. The current main stream technology for white treatment after forming the thin film. Otherwise, surface light emission couples blue LED and color-converted ligands should be short and conductive or could be ex- phosphors to produce white light. Commercially available changed to the desired type. The long and insulating phosphors show high quantum efficiency of above 90 % ligands should be avoided if post-treatment is not available for yellow (yttrium aluminum garnet, YAG) and 80 % for for conductivity enhancement. red (CaAlSiN, CASN). Quantum dots need more im- Compound toxicity and abundance could also be a provement in solid-state form to match those values. factor when it comes down to mass production of solar However, quantum dots could still serve as useful sub- cell materials. Toxicity of cadmium and lead has been a stitutes or add-ons of those phosphors for enhanced concern since the initial development of quantum dots. performance due to several reasons. First of all, their The issue could be managed if appropriate package and size-tunable emission property makes the multiple post-treatment are applied to the products. Most of the color emission easy to achieve without employing mul- compounds used in the current quantum dots are tiple compositions. This could reduce the chemical abundant for R&D and practical applications. Tellurium compatibility and aging problem when intermixture of mightbeaconcernasitislistedastheninerarest different compositions is inevitable. Second, the con- “metals” in the earth. So the mass production of tellurium- tinuous tunable color of quantum dot enables them to based quantum dots will be considered with caution, be added in to reach a high color rending index (CRI) which would need to be concerned by CdTe thin film solar value. By adding more phosphors with different colors, cell as well. thewhitelightsourceshowsrichercolor,broaderemis- Different morphologies of semiconductor nanocrystals sion spectrum, and higher CRI value. Third, nanocrys- also affect the solar cell device performance. In quantum tals are very light and tiny so they could relief the dot-based solar cells, charge separation happens at the phosphor settling problem due to the gravity. Finally, donor/acceptor interface, and charge carriers are trans- commercially available phosphors could be restricted ported in quantum dot phase by hopping and tunneling. If by the abundance of rare earth elements. Quantum dot semiconductor nanocrystals have one or two dimensions phosphor, if selected with right material systems, will of freedom (like nanorod), excitons can be separated along not face such kind of problems. the free dimension and free carriers can also be transported along the direction. A more ideal structure will be White Light Emission by Depositing Red-Emitting CdSe/ZnS an organized array of nanocrystals perpendicular to the QDs on a Blue/Green InGaN/GaN Quantum Well LEDs electrode surface. This will provide more interface area for CdSe MSQD has been reported as a potential phos- charge separation and significantly improve the charge phor [79]. The used MSQD has a significant amount transport efficiency. Post-synthesis techniques need to be of tail emission all over the visible range and is used designed to achieve such kind of structure. to simulate the white emission. By excitation from a UV light, a chromaticity coordinate of (0.324, 0.322) and a high CRI of 93 have been reported from the Solid-State Lightings and Displays emission. A potential problem of this approach origi- Quantum dot application in solid-state lighting and color nates from its emission mechanism. Most part of the display is very close to commercialization due to the su- spectrum comes from the defect emission which will perior optical properties of the quantum dots. Compared be hard to repeat for industry processing, and the to its immediate precedence organic semiconductor-based brightness will be greatly restricted as well. optical devices, it is chemically more stable and enables The requirements of quantum dot to serve as phosphor purer color emission. Compared to the traditional semi- is mainly the emission wavelength, quantum efficiency, conductor device, it is significantly lower in fabrication and the chemical stability. As an initial step, phosphors cost and compatible to flexible large substrates. with different colors (compared to the available phosphor) Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 11 of 15

can serve as add-ons to boost white light emission Successful QD-LEDs were first demonstrated by reaching higher CRI value. As a second step to re- Alivisatos group in 1994 [81]. TOPO-TOP-capped CdSe place main stream phosphors, quantum efficiencies QDs were used as the emission layer, and poly(p-phenyle- of quantum dot phosphors need to reach comparable nevinylene) (PPV) was used as hole transport layer. Differ- values with current competitors. The chemical stabil- ent color emissions from red to yellow were demonstrated ity at elevated temperature is also a key parameter although the external quantum efficiency is pretty low. to be evaluated which could be improved by selecting ap- The low quantum efficiency of early works is partially propriate surface ligands. Overall, this is the application of due to the imbalanced carrier injection into the emission quantum dots closest to commercialization due to the layer. The problem was addressed adding an electron simplicity of achieving photoluminescence. transport layer tri(8-hydroxyquinoline) (Alq3) to form The abovementioned quantum dot phosphors utilize the sandwich structure [82]. With the new device struc- an absorption-reemission process to emit light. This ture, Bawendi and Bulovic group have reported an exter- process is not efficient enough than another process nal quantum efficiency of >2 % and brightness of over called Förster resonance energy transfer (FRET). An ef- 7000 cd/m2 on their optimized device [83]. Besides the fective FRET requires two conditions: a short center-to- sandwich structure, those devices also feature monolayer center separation distance between the two interacting thickness of QD emission layer and a phase separation composites and a spectral overlap between donor emis- technique to separate emission layer and HTL during sion and acceptor absorption. The second condition is spin casting process. usually fulfilled for the quantum dot phosphor coupled The highest device brightness by far was reported by with traditional semiconductor LEDs. But to satisfy the Sun et al. in 2007 [84]. They used the similar device struc- first condition (distance within 10 nm), special architec- ture but optimized QD emission layer thickness based on tures are needed to bring quantum dot close enough to different QD size, structure, and emission colors. The the emission media. Klimov et al. put a monolayer of maximum brightness was reported to be 9064, 3200, 4470, CdSe/ZnS quantum dots on top of the InGaN quantum and 3700 cd/m2 for red, orange, yellow, and green QD- well to achieve a ~55 % light conversion efficiency [80]. LEDs. A lifetime (a slow decay to 50 % of initial value after Most of the quantum dots used in LED applications over 300 h) was reported at a brightness of over utilize electroluminescence rather than photolumines- 1100 cd/m2. Those QD-LEDs also show low turn-on cence. The LED device based on quantum dot electro- voltage, improved electroluminescence efficiency, and luminescence is called QD-LED. Extensive research efforts defect-free emission on relatively large surface areas. have been put on the device during the last decade. QD- Multi-color QD-LEDs can find their applications in LED utilizes a similar structure as OLED (organic LED) color displays. But among those blue emission is the and is composed of two electrodes for charge injection, most difficult to achieve due to the difficulties in obtain- two charge transport layers (for electron and hole trans- ing high-quality blue QD and the low sensitivity of portation), and the quantum dot emission layer. The elec- human eyes to blue light. QD size shrinks according to trodes are ITO for hole injection and metal for electron the blue shift of emission wavelength. As a result, QD injection for most of the cases. The charge transport layers with blue emission has the smallest size and usually are usually conducting polymer or organic molecules forms at the initial stage of QD growth. Thus, more de- which help to transport electron and holes into the fects are expected for blue QD which results in lower emission layer. quality. Also, the luminous efficacies (specifying the Electroluminescence of quantum dots usually comes average sensitivity of human eyes to light with different from two mechanisms: direct charge injection and Förster wavelength) of blue light are significantly lower (0.02–0.2) resonanceenergytransfer.Inthedirectchargeinjection than green and red (0.8–1), requiring much higher radiant case, electron and hole will be injected from two sides of power for blue light to achieve a brightness comparable to the electrodes and then trapped in quantum dots due to its red and yellow competitors. the energy alignment between electron transport layer Up to now, the best blue QD-LED was reported by (ETL), emission layer, and hole transport layer (HTL). The Tan et al. [85]. They used CdS/ZnS core/shell QD to trapped electron and hole will form exciton accordingly build the device, and ETL is omitted to avoid the extra and then recombine radiatively to emit light. In the Förster organic emission from Alq3. The device demonstrates energy transfer case, excitons will form on organic mole- pure blue emission with high brightness (1600 cd/m2), a cules close to QD, interstitial spaces, and voids in the QD low operating voltage (~5.5 V), and a narrow bandwidth layer. Due to fulfillment of the two requirements (spatial (~20 nm). Recent progress on the QD-LEDs is updated distance close enough and energy overlap), those excitons in review articles [94, 95]. will likely move to lower energy QD sites via FRET then Besides the single color QD-LED, QD electrolumines- undergo the radiative recombination. cence can also be used in white LED. Emission layer is Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 12 of 15

usually composed of several kinds of QD or a mixture of LED. Such kind of device has been demonstrated by QD with organic semiconductors. For example, a polymer Bulovic and Bawendi group at MIT [87]. In the device, the poly[(9,9-dihexyloxyfluoren-2,7-diyl)-alt-co-(2-methoxy-5- hole and electron transport layers are replaced by p-type phenylen-1,4-diyl)] (PFH-MEH) for blue, a CdSe/ZnS QD NiO and n-type ZnO:SnO2. The device demonstrates a for red color, and Alq3 for green have been combined peak brightness of 1950 cd/m2 and an external quantum together to achieve white emission [86]. In another work, efficiency of 0.1 %. A big challenge of the work is to select three different types of QDs (CdSe/ZnS for red, ZnSe/ an appropriate inorganic n-type semiconductor material CdSe/ZnS for green, and CdZnS alloy for blue) were to serve as ETL without damaging QD emission layer. used to form a mixed monolayer of QD for the white Room temperature radiofrequency (RF) sputtering was light emission. Besides N,N′-bis(3-methylphenyl)-N,N selected as the technique for depositing ETL and HTL to ′-bis(phenyl)benzidine (TPD) as the hole transport keep the QD layer intact. The two charge transport layers layer and Alq3 as electron transport layer, an additional were selected to have similar free-carrier concentration 3,4,5-triphenyl-1,2,4-triazole (TAZ) was added between and energy band offset to the QD for balanced charge in- QD emission layer and Alq3 as hole block layer. Blue jection. The resistivity and surface roughness of the two QD has the lowest quantum yield (40 %) among the layers were also tuned during deposition for good charge three colors. Furthermore, blue emission is unfavorable conductivity and electrical contact. The potential benefits compared to red and green by both direct injection of this approach are the following: it provides large selec- (higher energy barrier) and energy transfer (less spec- tion of materials and tunability of conductivity and energy tral overlap with TPD PL). To compensate and achieve band during processing. balanced emission, a much higher blue QD ratio was As claimed before, QD-LEDs are compatible to used in the mixed layer. The device shows a color rend- large and flexible substrates due to its solution pro- ing index of 86 which is comparable to commercial cessing property. An initial work of flexible QD-LED white LEDs with a maximum brightness of 830 cd/m2. was demonstrated on ITO-coated poly(ethylene-ter- The white QD-LED is much more complicated than ephthalate) (PET) substrates [88]. The device struc- single color QD-LED due to the involvement of much ture is similar with comparable brightness reported as more material systems and interactions in between. In device based on rigid substrates. The device flexibility the three-color mixing QD-LED example shown above, is demonstrated by remaining its performance until notonlydirectchargesintoQDsandenergytransfer bending radii reduces down to ~5 mm. This value is from organic molecules to QDs need to be considered below the bending limit of many possible applications but also energy transfers between QDs (from blue to and sufficient for large, roll-up flat panel displays. green, red and green to red) and from QD to organic Overall, LEDs based on QD electroluminescence face molecules (blue QD to Alq3) are important factors. A more technical challenges than devices using photolumi- detailed understanding of the whole system is the pre- nescence. But it provides much more freedom in device requisite for obtaining a white light source based on design and has more potential to achieve better perform- mixed QD electroluminescence. ance. Since it no longer requires the pumping source Furthermore, there are a few intrinsic problems needed made from traditional LED chip, the QD benefits could to be considered for white QD-LED design. Faster deg- be fully realized. The future of QD-LEDs can be pre- radation rate of organic semiconductors (OSC) will dicted by comparing it with OLEDs due to many similar- bring aging problem of the hybrid OSC/QD system and ities these two types of LEDs share. The first organic result in undesired spectrum shift over time. For mix- electroluminescence device was demonstrated in 1987 ing QD emission device, QD with high emission purity [89] and OLEDs have been extensively studied during is not desired due to the low CRI value it will bring. the last decade. OLED products have been commercial- The complication brought by more material system ized in cell phone, display, and many other applications. and additional interactions also adds more technical The QD-LEDs have some intrinsic benefits over OLEDs challenges than single color QD-LEDs. Although the (chemical stability, emission properties, etc.) and can white QD-LED has a long way to commercialization benefit from OLED technique due to their similar struc- and even cannot compete with white LED using QD as ture, so the commercialization of QD-LEDs should be phosphor, it also has its unique potential in terms of seen in the near future. offering large area and flexible lighting source which There are a few common properties to be shared be- will render it an interesting direction to be explored in tween QDs used in solar cell and those in LEDs: they the next few decades. should have good conductivity and chemical stability, Material degradation and electrode contact are two lim- abundant amount of supply, and controllable toxicity. iting factors of QD-LED lifetime. To address the problem, QDs in LEDs undergo more hush conditions due to the efforts have been put on building pure inorganic QD- elevated temperature from high injection current. Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 13 of 15

(1)A high quantum efficiency (high PL usually leads Authors’ Information to high EL) indicates good recombination and low No additional info. defect level. (2)There are several reasons for reduced EL efficiency Acknowledgements Authors thank the sponsorship by the Oak Ridge National Laboratory, compared to PL efficiency: liquid to solid lost (one Laboratory Directed Research and Development (LDRD) program. magnitude), inefficient and imbalanced charge injection. Received: 5 October 2015 Accepted: 22 November 2015 (3)Emission bandgap can be designed to fulfill specific wavelength. (4)The narrower the bandwidth the better, for single References 1. Masala O, Seshadri R (2004) Synthesis routes for large volumes of color emission and color display. But for white nanoparticles. Annu Rev Mater Res 34:41–81 LEDs, high color purity is not desirable due to the 2. Cao YC, Wang JH (2004) One-pot synthesis of high-quality zinc-blende low CRI it will bring. CdS nanocrystals. J Am Chem Soc 126(44):14336–14337 3. Yang YA et al (2005) Synthesis of CdSe and CdTe nanocrystals without (5)Good conductivity is desirable. precursor injection. Angew Chem Int Ed Engl 44(41):6712–6715 (6)Good chemical and thermal stability are required 4. Ouyang J et al (2008) Multiple families of magic-sized CdSe nanocrystals due to high current and elevated operating with strong bandgap photoluminescence via noninjection one-pot syntheses. J Phys Chem C 112(36):13805–13811 temperature. Good encapsulation could help 5. Yu K et al (2009) Single-sized CdSe nanocrystals with bandgap photoemission maintain the stability. The encapsulation via a non-injection one-pot approach. J Phys Chem C 113(9):3390–3401 technique developed from OLEDs could be 6. Ouyang JY et al (2009) Photoluminescent colloidal CdS nanocrystals with high quality via non-injection one-pot synthesis in 1-octadecene. J Phys applicable for the LEDs. Chem C 113(18):7579–7593 (7)Low toxicity is necessary for environmental compliance 7. Liu TY et al (2009) Non-injection and low-temperature approach to colloidal by replacement of CdSe QDs with non-toxic elements. photoluminescent PbS nanocrystals with narrow bandwidth. J Phys Chem C 113(6):2301–2308 8. Ouyang JY et al (2009) Noninjection, one-pot synthesis of photoluminescent Finally, surface engineering may play a significant role colloidal homogeneously alloyed CdSeS quantum dots. J Phys Chem C for better charge injection, lower nonradiative recombin- 113(13):5193–5200 9. Wang R et al (2009) Homogeneously-alloyed CdTeSe single-sized nanocrystals ation, and improved thermal stability. Design of novel with bandgap photoluminescence. J Phys Chem C 113(9):3402–3408 material system with comparable or better performance 10. Wang RB et al (2009) Magic-sized Cd3P2 II-V nanoparticles exhibiting than cadmium-based QD is needed. bandgap photoemission. J Phys Chem C 113(42):17979–17982 11. Revaprasadu N et al (1999) Novel single-molecule precursor routes for the direct synthesis of InS and InSe quantum dots. J Mater Chem 9(11):2885–2888 Conclusions 12. Malik MA, Revaprasadu N, O’Brien P (2001) Air-stable single-source precursors for the synthesis of chalcogenide semiconductor nanoparticles. This paper has reviewed the various experimental Chem Mater 13(3):913–920 methods in the laboratory scale. It appears that the non- 13. Li ZG, Cai W, Sui JH (2008) Large-scale preparation of CdS quantum dots by direct thermolysis of a single-source precursor. Nanotechnology 19(3): injection one-pot synthesis route seems to have the 035602 promise for scale-up engineering development for larger 14. Cumberland SL et al (2002) Inorganic clusters as single-source precursors for scale production with a chemical engineering stir-tank preparation of CdSe, ZnSe, and CdSe/ZnS nanomaterials. Chem Mater 14(4): 1576–1584 reactor. This paper also reviewed a few limited literature 15. Yin Y, Alivisatos AP (2005) Colloidal nanocrystal synthesis and the organic– developments on scale-up engineering development and inorganic interface. Nature 437(7059):664–670 green approaches. Applications in energy such as solar 16. Yu K et al (2010) Thermodynamic equilibrium-driven formation of single- sized nanocrystals: reaction media tuning CdSe magic-sized versus regular cells and solid-state lightings demand quantum dots as quantum dots. J Phys Chem C 114(8):3329–3339 “commodity” materials. The challenges on large quantity 17. Ithurria S, Dubertret B (2008) Quasi 2D colloidal CdSe platelets with production while maintaining the high optical quality of thicknesses controlled at the atomic level. J Am Chem Soc 130(49):16504 18. Son JS et al (2009) Large-scale soft colloidal template synthesis of 1.4 nm quantum dots need to be addressed. thick CdSe nanosheets. Angewandte Chemie-International Edition 48(37): 6861–6864 Abbreviations 19. Peng ZA, Peng XG (2002) Nearly monodisperse and shape-controlled CdSe CRI: color rending index; ETL: electron transport layer; FRET: Förster nanocrystals via alternative routes: nucleation and growth. J Am Chem Soc resonance energy transfer; HTL: hole transport layer; PL: photoluminescence; 124(13):3343–3353 QDs: quantum dots; LEDs: light-emitting diodes; OLEDs: organic light-emitting 20. Jiang ZJ, Kelley DF (2010) Role of magic-sized clusters in the synthesis of diodes. CdSe nanorods. ACS Nano 4(3):1561–1572 21. Kasuya A et al (2004) Ultra-stable nanoparticles of CdSe revealed from mass spectrometry. Nat Mater 3(2):99–102 Competing Interests 22. Arellano IHJ et al (2010) 3-component low temperature solvothermal synthesis The authors declare that they have no competing interests. of colloidal cadmium sulfide quantum dots. Mater Lett 64(6):785–788 23. Wang QA et al (2006) Luminescent CdSe and CdSe/CdS core-shell Authors’ contributions nanocrystals synthesized via a combination of solvothermal and two-phase MZH continued to rewrite and refine the manuscript that was originally thermal routes. J Lumin 118(1):91–98 drafted by TZ who was formerly a postdoc at the Oak Ridge National 24. Xu J, Ge J-P, Li Y-D (2006) Solvothermal synthesis of monodisperse PbSe Laboratory. All authors read and approved the final manuscript. nanocrystals. J Phys Chem B 110(6):2497–2501 Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 14 of 15

25. Nag A, Sarma DD (2009) Solvothermal synthesis of InP quantum dots. 53. Wang H et al (2005) Highly luminescent CdSe/ZnS nanocrystals synthesized J Nanosci Nanotechnol 9(9):5633–5636 using a single-molecular ZnS source in a microfluidic reactor. Adv Funct 26. Nakamura H et al (2006) Tunable photoluminescence wavelength of Mater 15(4):603–608 chalcopyrite CuInS2-based semiconductor nanocrystals synthesized in a 54. Nuchter M et al (2004) Microwave assisted synthesis—a critical technology colloidal system. Chem Mater 18(14):3330–3335 overview. Green Chem 6(3):128–141 27. Li T-L, Teng H (2010) Solution synthesis of high-quality CuInS2 quantum dots 55. Li L, Qian H, Ren J (2005) Rapid synthesis of highly luminescent CdTe as sensitizers for TiO2 photoelectrodes. J Mater Chem 20(18):3656–3664 nanocrystals in the aqueous phase by microwave irradiation with 28. Kalasad MN, Rabinal AK, Mulimani BG (2009) Ambient synthesis and controllable temperature. Chem Commun 4:528–530 characterization of high-quality CdSe quantum dots by an aqueous 56. He Y et al (2008) Microwave synthesis of water-dispersed CdTe/CdS/ZnS core- route. Langmuir 25(21):12729–12735 shell-shell quantum dots with excellent photostability and biocompatibility. 29. Guo J, Yang W, Wang C (2005) Systematic study of the photoluminescence Adv Mater 20(18):3416–3421 dependence of thiol-capped CdTe nanocrystals on the reaction conditions. 57. Chu MQ, Shen XY, Liu GJ (2006) Microwave irradiation method for the J Phys Chem B 109(37):17467–17473 synthesis of water-soluble CdSe nanoparticles with narrow photoluminescent 30. Zhou L et al (2010) One-pot large-scale synthesis of robust ultrafine silica- emission in aqueous solution. Nanotechnology 17(2):444–449 hybridized CdTe quantum dots. ACS Appl Mater Interfaces 2(4):1211–1219 58. Qian H et al (2006) Microwave-assisted aqueous synthesis: a rapid approach 31. Law WC et al (2009) Aqueous-phase synthesis of highly luminescent CdTe/ to prepare highly luminescent ZnSe(S) alloyed quantum dots. J Phys Chem ZnTe Core/Shell quantum dots optimized for targeted bioimaging. Small B 110(18):9034–9040 5(11):1302–1310 59. Qian H, Li L, Ren J (2005) One-step and rapid synthesis of high 32. Zeng QH et al (2008) Synthesis and optical properties of type II CdTe/CdS quality alloyed quantum dots (CdSe-CdS) in aqueous phase by core/shell quantum dots in aqueous solution via successive ion layer microwave irradiation with controllable temperature. Mater Res adsorption and reaction. J Phys Chem C 112(23):8587–8593 Bull 40(10):1726–1736 33. Xia YS, Zhu CQ (2008) Aqueous synthesis of type-II core/shell CdTe/CdSe 60. Gerbec JA et al (2005) Microwave-enhanced reaction rates for nanoparticle quantum dots for near-infrared fluorescent sensing of copper(II). Analyst synthesis. J Am Chem Soc 127(45):15791–15800 133(7):928–932 61. Washington AL II, Strouse GF (2008) Microwave synthesis of CdSe and 34. Li H, Shih WY, Shih WH (2007) Non-heavy-metal ZnS quantum dots CdTe nanocrystals in nonabsorbing alkanes. J Am Chem Soc with bright blue photoluminescence by a one-step aqueous synthesis. 130(28):8916–8922 Nanotechnology 18:205604 62. Washington AL, Strouse GF (2009) Microwave synthetic route for highly 35. Zheng YG, Yang ZC, Ying JY (2007) Aqueous synthesis of glutathione-capped emissive TOP/TOP-S passivated CdS quantum dots. Chem Mater 21(15): – ZnSe and Zn1 − xCdxSe alloyed quantum dots. Adv Mater 19(11):1475 3586 3592 36. Lan GY et al (2007) Photo-assisted synthesis of highly fluorescent ZnSe(S) 63. Ziegler J et al (2007) High-quality ZnS shells for CdSe nanoparticles: rapid quantum dots in aqueous solution. J Mater Chem 17(25):2661–2666 microwave synthesis. Langmuir 23(14):7751–7759 37. Zhang H et al (2003) Hydrothermal synthesis for high-quality CdTe 64. Rossetti R, Nakahara S, Brus LE (1983) Quantum size effects in the redox nanocrystals. Adv Mater 15(20):1712–1715 potentials, resonance Raman spectra, and electronic spectra of CdS – 38. Ge JP et al (2006) Synthesis of CdSe, ZnSe, and ZnxCd1 − xSe nanocrystals crystallites in aqueous solution. J Chem Phys 79(2):1086 1088 and their silica sheathed core/shell structures. Inorg Chem 45(13):4922–4927 65. Talapin DV et al (2009) Prospects of colloidal nanocrystals for electronic and 39. Wang X et al (2005) A general strategy for nanocrystal synthesis. Nature optoelectronic applications. Chem Rev 110(1):389–458 437(7055):121–124 66. Elif Arici NSS (2004) Hybrid solar cells. Encyclopedia Nanosci Nanotechnol 40. Liu LP, Peng Q, Li YD (2008) Preparation of CdSe quantum dots with full 3:929–944 color emission based on a room temperature injection technique. Inorg 67. Kumar S, Scholes GD (2008) Colloidal nanocrystal solar cells. Microchim Acta Chem 47(11):5022–5028 160(3):315–325 41. Li M et al (2007) Hydrothermal synthesis of highly luminescent CdTe 68. Kamat PV (2013) Emergence of quantum dot solar cells as next generation quantum dots by adjusting precursors’ concentration and their conjunction photovoltaics. Amer Chem Soc 246:18737–18753. with BSA as biological fluorescent probes. Talanta 72(1):89–94 69. Dai Q, Duty CE, Hu MZ (2010) Semiconductor-nanocrystals-based white 42. Yang W, Li W, Dou H, Sun K (2008) Hydrothermal synthesis for high- light-emitting diodes. Small 6(15):1577–1588 quality CdTe quantum dots capped by cysteamine. Mater Lett 70. Shockley W, Queisser HJ (1961) Detailed balance limit of efficiency of p-n 62(17–18):2564–2566 junction solar cells. J Appl Phys 32(3):510–519 43. Zhao D et al (2008) Synthesis and characterization of high-quality water- 71. Johnston KW et al (2008) Schottky-quantum dot photovoltaics for efficient soluble near-infrared-emitting CdTe/CdS quantum dots capped by N-acetyl- infrared power conversion. Appl Phys Lett 92(15):151115–3 l-cysteine via hydrothermal method. J Phys Chem C 113(4):1293–1300 72. Tang J et al (2010) Schottky quantum dot solar cells stable in air under solar 44. Wang J, Han H (2010) Hydrothermal synthesis of high-quality type-II illumination. Adv Mater 22(12):1398–1402 CdTe/CdSe quantum dots with near-infrared fluorescence. J Colloid 73. Luther JM et al (2008) Schottky solar cells based on colloidal nanocrystal Interface Sci 351(1):83–87 films. Nano Lett 8(10):3488–3492 45. Mao WY, Guo J, Yang W, Wang C, He J, Chen J (2007) Synthesis of high- 74. Ma W et al (2009) Photovoltaic devices employing ternary PbSxSe1 − x quality near-infrared-emitting CdTeS alloyed quantum dots via the nanocrystals. Nano Lett 9(4):1699–1703 hydrothermal method. Nanotechnology 18(48):7 75. Clifford JP et al (2007) Schottky barriers to colloidal quantum dot films. 46. Kawa M et al (2003) Large-scale production of CdSe nanocrystal by a Appl Phys Lett 91(25):253117–3 continuous flow reactor. J Nanoparticle Res 5(1):81–85 76. Gur I et al (2005) Air-stable all-inorganic nanocrystal solar cells processed 47. Murray CB, Norris DJ, Bawendi MG (1993) Synthesis and characterization of from solution. Science 310(5747):462–465 nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor 77. Kamat PV (2008) Quantum dot solar cells. Semiconductor nanocrystals as nanocrystallites. J Am Chem Soc 115(19):8706–8715 light harvesters†. J Phys Chem C 112(48):18737–18753 48. Dabbousi BO et al (1997) (CdSe)ZnS core-shell quantum dots: synthesis 78. Lee HJ et al (2008) CdSe quantum dot-sensitized solar cells exceeding and characterization of a size series of highly luminescent nanocrystallites. efficiency 1 % at full-sun intensity. J Phys Chem C 112(30):11600–11608 J Phys Chem B 101(46):9463–9475 79. Schreuder MA et al (2008) Encapsulated white-light CdSe nanocrystals as 49. Peng X et al (1997) Epitaxial growth of highly luminescent CdSe/CdS Core/ nanophosphors for solid-state lighting. J Mater Chem 18(9):970–975 Shell nanocrystals with photostability and electronic accessibility. J Am Chem 80. Achermann M et al (2004) Energy-transfer pumping of semiconductor Soc 119(30):7019–7029 nanocrystals using an epitaxial quantum well. Nature 429(6992):642–646 50. Peng X et al (2000) Shape control of CdSe nanocrystals. Nature 404(6773):59–61 81. Colvin VL, Schlamp MC, Alivisatos AP (1994) Light-emitting diodes made 51. Chan EM, Mathies RA, Alivisatos AP (2003) Size-controlled growth of CdSe from cadmium selenide nanocrystals and a semiconducting polymer. nanocrystals in microfluidic reactors. Nano Lett 3(2):199–201 Nature 370(6488):354–357 52. Wang H et al (2004) Continuous synthesis of CdSe-ZnS composite 82. Coe S et al (2002) Electroluminescence from single monolayers of nanoparticles in a microfluidic reactor. Chem Commun 1:48–49 nanocrystals in molecular organic devices. Nature 420(6917):800–803 Hu and Zhu Nanoscale Research Letters (2015) 10:469 Page 15 of 15

83. Coe-Sullivan S et al (2005) Large-area ordered quantum-dot monolayers via phase separation during spin-casting. Adv Funct Mater 15(7):1117–1124 84. Sun Q et al (2007) Bright, multicoloured light-emitting diodes based on quantum dots. Nat Photon 1(12):717–722 85. Tan Z et al (2007) Bright and color-saturated emission from blue light- emitting diodes based on solution-processed colloidal nanocrystal quantum dots. Nano Lett 7(12):3803–3807 86. Li Y et al (2005) White organic light-emitting devices with CdSe/ZnS quantum dots as a red emitter. J Appl Phys 97(11):113501–113504 87. Caruge JM et al (2008) Colloidal quantum-dot light-emitting diodes with metal-oxide charge transport layers. Nat Photon 2(4):247–250 88. Tan Z et al (2009) Colloidal nanocrystal-based light-emitting diodes fabricated on plastic toward flexible quantum dot optoelectronics. J Appl Phys 105(3):034312–034315 89. Tang CW, VanSlyke SA (1987) Organic electroluminescent diodes. Appl Phys Lett 51(12):913–915 90. Li JJ et al (2003) Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction. J Am Chem Soc 125(41):12567–12575 91. Yu WW et al (2003) Experimental determination of the extinction coefficient of CdTe, CdSe, and CdS nanocrystals. Chem Mater 15(14):2854–2860 92. Chen L et al (2014) Recent advances in the use of near-infrared quantum dots as optical probes for bioanalytical, imaging and solar cell application. Microchim Acta 181(13–14):1485–1495 93. Pichaandi J et al (2014) Near-infrared emitting quantum dots: recent progress on their synthesis and characterization. Coord Chem Rev 263(138–150):138–150 94. Liu Y et al (2015) Research progress on high-efficiency and stable II–VI group quantum-dot light-emitting diodes. Chin J Inorg Chem 31(9):1751–1760 95. McKittrick J et al (2014) Review: down conversion materials for solid-state lighting. J Am Ceram Soc 97(5):1327–1352

Submit your manuscript to a journal and beneﬁ t from:

7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the ﬁ eld 7 Retaining the copyright to your article

Submit your next manuscript at 7 springeropen.com